Method of preserving mechanical properties of steel wire during spot welding

ABSTRACT

A method for preserving the mechanical properties of steel wire or rod during spot welding of such steel wire or rod to form cages for use in prestressed concrete. The steel reinforcing rods are heated to a temperature of from 300-450* C. for from 5 seconds to 15 minutes after being spot welded.

United States Patent I2 1 I I Tatuo Amakasu Tokyo;

Takao Yamazaki, Fujisawa-shi, Japan 781,174

Dec. 4, 1968 Apr. 6, 1971 Koushuha-netsuren Kabushiki Kaisha Tokyo, Japan Inventors Appl. No. Filed Patented Assignee METHOD OF PRESERVING MECHANICAL PROPERTIES OF STEEL WIRE DURING SPOT WELDING 4 Claims, 6 Drawing Figs.

US. Cl 219/58, 148/127 Int. Cl C2ld 1/30 Field otSearch 2l9/56, 57,

ELONGATION [56] References Cited UNITED STATES PATENTS 1,298,590 3/1919 Smith 219/60X 2,314,691 3/1943 Dawson et a1. 219/114 3,015,018 12/1961 Rudd 219/67 Primary Examiner.l. V. Truhe Assistant ExaminerHugh D. Jaeger Att0rneyWender0th, Lind and Ponack ABSTRACT: A method for preserving the mechanical properties of steel wire or rod during spot welding of such steel wire or rod to form cages for use in prestressed concrete. The steel reinforcing rods are heated to a temperature gt frong lifl);

450 C. for from 5 seconds to 15 minutes after being spot welded.

FIG. Ic

5 FIG. l0.

TA'IUQ AMAKASU and TAKAO YAMAZAKI,

INVENTOR 5 METHOD OF PRESERVING MECHANICAL PROPERTIES OF STEEL WIRE DURING SPOT WELDING The present invention relates to a method of preserving the mechanical properties of steel reinforcing wire or rod during spot welding, by preventing deterioration or recovering the lost mechanical properties of the welded parts of said steel wire or rod. More particularly, this invention relates to a method for preserving, by recovering, or preventing loss of mechanical properties of the welded parts of steel reinforcing wire or rod used in the formation of steel wire cages to be embedded in concrete for the purpose of enhancing the strength of concrete structural products such as concrete pipes, poles, piles, tanks, etc. in prestressed concrete. Such loss of proper ties occurs when spot welding the contacting parts of the steel rods and hoop-reinforcing wires, said steel rods being arranged parallel to each other and at certain intervals according to the external shape of the concrete structure, and said hoop-reinforcing wires being fixed spirally on the outer circumference of said steel rod arrangement.

In manufacturing prestressed concrete structures, hitherto, the method which has been generally adopted is to arrange the required number of reinforcing steel rods parallel to each other and at certain intervals, according to the external shape of the structure, such as a concrete pillar or the like which will have the rods incorporated therein. Then steel reinforcing wires or the like are wound spirally around the outer circumference of the steel rod group thus formed. To fix and secure the contacting parts of said steel rods and said hoop-reinforcing wires, they are bound manually with fine steel wires, thus forming a columnar steel wire cage. Said cage is set in the concrete mold in the proper position, and tension is applied to said steel reinforcing rods while concrete is poured into the mold. This is followed by introducing the necessary stress into the concrete with the steel rods and placing the mold in a centrifugal pillar manufacturing machine or the like to string it up. It is then cured in steam, and the mold is removed. It is then cured in water and in air, and a prestressed concrete pillar is obtained having satisfactory strength and into which the stress has been introduced by the steel rods.

An automatic forming machine has recently been introduced to make highly efficient the production of such a steel reinforcing wire cage that is an essential element for enhancing the strength of such a concrete product.

The object of this invention lies mainly in making it possible to form advantageously and with high efficiency steel wire cages composed of reinforcing steel rods, which cages have excellent mechanical properties.

Other objects and advantages of the invention and its valuable characteristics will become apparent from the following description taken in connection with the accompanying drawings, in which:

FIGS. Ia and 1b are side and end elevation views, respectively, showing a steel reinforcing wire cage;

FIG. la is a side elevation view of a reinforcing wire cage with means for supplying current to the rods;

FIGS. 2a and 2b are graphs showing changes in the mechanical properties of steel reinforcing rods which have been heated for certain times and at temperatures from 300 C. to 500 C. prior to, as well as subsequent to, the spot welding of the contacting parts of the hoop-reinforcing wires to said steel rods. FIG. 20 showing the change in tensile strength, and FIG. 2b showing the elongation; and

FIG. 3 is a graphic representation of the distribution of the hardness in the welded part of the steel reinforcing rods which have been heated for at least 5 minutes at at least 350 C. after spot welding the contacting parts of the rods and the hoopreinforcing wires.

The above-described efficient method of forming steel reinforcing rod cages by using an automatic forming machine will be illustrated with reference to FIG. 1. The required number of steel reinforcing rods 1 are arranged parallel to each other with their ends fixed to a holding means such as an anchor plate 5. At spaced points along the rods there is positioned within the inner circumference of the rods a drum 4 or the like. The group of steel rods can move and revolve continuously along the outer circumference of said drum 4. Around the outer circumference of the group of steel rods is spirally wound hoop-reinforcing wires 2 made of steel. This is done by relatively rotating the rods and the wire, i.e. by fixing either the group of steel rods or the hoop-reinforcing wires 2 and rotating the other. The points 3, where the respective steel rods 1 and the hoop-reinforcing wires 2 contact each other, are successively and automatically spot welded by an electrode 6 for spot welding, which electrode is connected to a power source.

The rods generally are of a steel having a carbon content of from 0.20.65. Typical compositions fall within the ranges of Table I.

TABLE I 0, 02-065; Si, 0.152.5; Mn, 0.60-0.90; P, 0.030;

Typical compositions of the hoop-reinforcing wires are shown shown in Table II.

TABLE II In so far as the formation of this kind of steel wire cage by this method is extremely efficient, said method is superior in comparison to conventional methods. However, a serious shortcoming of said method is that, when the rods 1 and wire 2 are welded, the mechanical properties of the welded parts of the rods deteriorate because said welded parts of the steel rods are suddenly heated and suddenly cooled off.

According to the experimental results obtained by Applicants in this respect, the values of the tensile strength and elongation of steel reinforcing rods 1 which have not been spot welded are as indicated by dots c in FIGS. 2a and 2b, respectively, while the values of the tensile strength and elongation of the same steel rods measured after the hoop-reinforcing wires 2 are mechanically separated from the steel rods 1 after having been spot welded thereto is indicated by dots d in the same FIGS. As can be seen, the mechanical properties of the steel reinforcing rods 1 will be changed so that the tensile strength will be reduced about 2 percent, and the elongation will be reduced about 50 percent. These reductions were substantially similar to those obtained in other experiments of the same kind. According to the results of Applicants research and experiments, this can be attributed to the fact that a partial metalographical-structural change is brought about in the welded parts of the steel reinforcing rod which contains carbon in amount up to the level of medium carbon steel or high carbon steel. The change in the metalographical structure of the welded portion is due to the fact that it is heated suddenly and quickly by the heat generated during spot welding and thereafter is suddenly and quickly cooled off. A serious consequence of such a structural change is that it will in its turn adversely influence the full achievement of the purpose of this kind of steel wire cage by failure to introduce sufficient stress into the concrete structure to enhance the strength thereof.

After all the various experiments and studies carried out in an attempt to find a solution to this difficulty, Applicants have finally discovered that, after the contacting parts of the steel reinforcing rods and the hoop-reinforcing wire have been spot welded, the welded parts should be heated to a temperature of from 300 C. to 450 C. by direct electrification or by other heating means.

Heating by direct electrification of the rods is carried out as shown in FIG. 10. Steel reinforcing rods 1 reinforced by hoopreinforcing wire 2 are spot welded thereto at 3. Electric con tact members 8 and 8' are electrically connected to a power source E, said contact members having a body with projections thereon adapted to be pressed against the end face of each rod. Hoop-reinforcing wires 2 re wound around the outer periphery of steel reinforcing rods 1 held in an appropriate jig or the like, a cage is formed by spot welding the contacting points 3. After removing the jig and before arranging the cage in the concrete mold. the end face of each of the steel rein forcing rods is contacted and held between said contact members 8 and 8'. Electricity is fed to the contact members 8 and 8 from the power source E. Thereupon, the electric current flows through each rod so as to enable the welded portion of the rod to be heated up to the desired temperature, i.e. 300- 450 C. When the heating to the desired temperature and for the desired time is completed, the current from the power source E is cut off and the contact members 8 an 8 are moved away from the end face of each rod. ln this way, it is possible to heat the welded portion of the rod to the desired temperature, and for the desired time.

The effectiveness of heating the welded parts up to about 300 C. to 450 C. by means of direct electrification or the like after spot welding them can be seen from FlGS. 2a and 2b, which show a part of the experimental results obtained by Applicants to demonstrate said effectiveness. FIGS. 2a and 2b show the change of tensile strength and elongation of steel reinforcing rods in the cases where steel reinforcing rods which have undergone no spot welding are heated to temperatures of from 300 to 500 C., and kept at said temperatures for 5 minutes, and where steel reinforcing rods of the same material, but which have been spot welded are similarly heated and kept at said temperatures for 5 minutes and I5 minutes, respectively. As shown in the FlGS., the steel reinforcing rods which have undergone no spot welding had the values as shown with dots c for both tensile strength and elongation, and after they had been kept for 5 minutes at temperatures from 300 to 500 C., the changes were as indicated by doted lines c. The steel reinforcing rods which had been spot welded had the values as shown by dots before heating. but after they had been heated to temperatures from 300 to 500 C., at which they were kept for 5 minutes, the changes were as shown by solid lines d. The steel rods which had been spot welded and ten heated to temperatures from 300 to 500 C. and kept at said temperatures for seconds had strengths and elongations as shown by dotted lines 11", while rods which had been spot welded and then heated to like temperatures for 5 seconds had strengths and elongations as shown by dotted lines 11". It will be seen from FlGS. 2a and 2h that the mechanical property of elongation of the steel reinforcing rods, which degenerated acutely due to spot welding, is restored fully to the value prior to the spot welding. It can be seen particularly with respect to curves d' and d"" that the elongation property of the steel reinforcing rods can be rapidly restored by heating for 20 seconds at a temperature of about 300 C., and can be similarly restored to a great extend by heating for 5 seconds at a temperature above 400 C.

The lower part of FIG. 3 is a representation of the changed metalographical structure of the part of the steel rods affected by spot welding. In said representation, the shaded part e is a portion of the hoop-reinforcing iron wire which has been welded to the corresponding surface part of the' steel rods, f is the part of the steel rod which has undergone metalographical change due to spot welding, and g is the portion which has undergone no such metalographical change. The curves in the upper part of FIG. 3 show the hardness distribution, as measured by a Vickers hardness tester with a load of 500g, in the respective parts along the section line h-h. The solid line shows the hardness distribution in the welded parts of the steel rods which have not been heated, and the dotted line shows the hardness in the welded parts of steel rods which have been heated at 350 C for 5 minutes after being spot welded. From FIG. 3 it will be seen that the hardness produced by the spot welding in the welded parts of the steel rods had been reduced substantially to that of the portion that has not been afiected by spot welding, and thus the hardness has been made substantiall uniform b the 5 minutes of heating at 350 C.

A though Fl l'shows a particular example of a method of forming one kind of steel wire cage, and the method has been described as applicable thereto, the present invention is also applicable to other ways of forming steel wire cages by spot welding the parts of the steel reinforcing rods and hoop-reinforcing wires where they contact each other. lt should therefore be understood that the invention is not limited to the particular arrangements disclosed hereinabove, but that the appended claims are intended to cover all modifications which do not depart from the true spirit and scope of the invention.

We claim:

I. A method of preventing deterioration of the mechanical properties of the welded parts of steel reinforcing rods due to spot welding during the formation of steel'wire cages for manufacturing prestressed concrete structures, comprising heating the welded parts of said rods to a temperature of from about 300 to about 430 C. for about 5 to about 20 seconds after the spot welding is completed.

2. A method as claimed in claim 1 in which said welded parts are heated to a temperature of from 420 to 430C.

37 A method as claimed in claim 1 in which said welded parts are heated to a temperature of from 300 to about 430 C. for 20 seconds.

4. A method of preventing deterioration of the mechanical properties of the welded parts of steel reinforcing rods due to spot welding during the formation of steel wire cages for manufacturing prestressed concrete structures, comprising heating the welded parts of said rods to a temperature of from about 300 to about 430 C. for 20 seconds to 5 minutes after the spot welding is completed. 

1. A method of preventing deterioration of the mechanical properties of the welded parts of steel reinforcing rods due to spot welding during the formation of steel wire cages for manufacturing prestressed concrete structures, comprising heating the welded parts of said rods to a temperature of from about 300* to about 430* C. for about 5 to about 20 seconds after the spot welding is completed.
 2. A method as claimed in claim 1 in which said welded parts are heated to a temperature of from 420* to 430* C.
 3. A method as claimed in claim 1 in which said welded parts are heated to a temperature of from 300* to about 430* C. for 20 seconds.
 4. A method of preventing deterioration of the mechanical properties of the welded parts of steel reinforcing rods due to spot welding during the formation of steel wire cages for manufacturing prestressed concrete structures, comprising heating the welded parts of said rods to a temperature of from about 300* to about 430* C. for 20 seconds to 5 minutes after the spot welding is completed. 